The invention provides a platinum-ruthenium catalyst for PEM fuel cells with a high tolerance toward poisoning by carbon monoxide. The catalyst is particularly suitable as an anode catalyst for fuel cells with a polymer electrolyte membrane (PEM fuel cells), but it is also suitable as an anode catalyst for direct methanol fuel cells (DMFC).
In principle, fuel cells are gas-operated batteries in which the energy obtained from the reaction of water and oxygen is converted directly into electrical energy. The present invention describes catalysts for PEM fuel cells (PEM=polymer electrolyte membrane) that are suitable for operation with hydrogen-containing gases or with methanol (DMFC=direct methanol fuel cell). The first-mentioned type of fuel cell is gaining in importance as a source of power for motor vehicles operated by electric engines, due to its high energy density and robustness, the latter type of fuel cell enables a reduction in the number of units required because a hydrogen-producing unit is not needed.
When compared with conventional internal combustion engines, fuel cells have very low emissions and, at the same time, very high efficiency. If hydrogen is used as a fuel gas, water is the only emission on the cathode side of the cell. Motor vehicles with such a drive system are called ZEVs (zero emission vehicles).
Currently, however, hydrogen is still too expensive and causes problems with regard to storage and the refuelling of vehicles. For these reasons, the alternative, producing hydrogen from methanol directly on-board the vehicle, is gaining in importance. The methanol stored in the vehicle tank is either converted into a hydrogen-rich fuel gas with carbon dioxide and carbon monoxide as secondary constituents, in a unit upstream of the fuel cell, or is oxidised directly on the anode of the fuel cell. In the former case, purification steps to react the carbon monoxide by a shift reaction or by preferential oxidation (PROX) are also required. Thus, in theory, the reformate gas consists of only 75 vol.-% hydrogen and 25 vol.-% carbon dioxide. In practice, this gas still contains some nitrogen and, depending on the degree of purification, variable amounts of carbon monoxide (up to 1 vol.-%). In the case of DMFC operation, the methanol is electrochemically oxidised directly on the anode to give carbon dioxide. An intermediate step in the complicated reaction mechanism of the anode reaction leads to adsorbed carbon monoxide, which is then oxidised further to carbon dioxide.
Catalysts based on platinum are used on the anode and cathode sides of PEM fuel cells. These consist of fine noble metal particles that are deposited on a conductive support material (generally carbon black or graphite). The concentration of noble metal is typically between 10 and 80 wt.-%, with respect to the total weight of the catalyst.
Traditional platinum catalysts are very sensitive to poisoning by carbon monoxide. Therefore, the concentration of carbon monoxide in the anode gas needs to be reduced to less than 10 ppm in order to prevent performance losses in the fuel cells due to poisoning of the anode catalyst. This applies in particular to PEM fuel cells which, with low working temperatures of up to 100xc2x0 C., are especially sensitive to poisoning by carbon monoxide. Even larger problems occur with DMFC systems.
The problem of the poisoning of platinum catalysts by carbon monoxide has been recognised for some time. Due to its special molecular structure, carbon monoxide is adsorbed on the surface of platinum thereby blocking access to the catalytically active centres in the platinum by hydrogen molecules in the anode gas.
It is also known that the tolerance of a platinum catalyst to poisoning by carbon monoxide can be improved by alloying or doping the platinum with other metals, for example ruthenium. Generally, this means that oxidation of the carbon monoxide adsorbed on the platinum to carbon dioxide takes place and the carbon dioxide is then readily desorbed. The oxygen required for this is supplied either in the form of small amounts of air (called an air-bleed in the following) or in bonded form as water in the anode gas stream. The use of an air-bleed leads, in addition to the oxidation of carbon monoxide, to the oxidation of some of the hydrogen, and thus, reduces the efficiency of the overall system. For this reason, catalysts for reduced air-bleed, or even air-bleed-free fuel cell systems are gaining in importance.
EP 0889 188 A2 describes a CO-tolerant PtRu anode catalyst in which the two noble metals are not alloyed with each other. The particle size of the platinum is less than 2 nm, that of the ruthenium is less than 1 nm. The high carbon monoxide tolerance is achieved with an air-bleed of 3 vol.-%. In addition, surface analysis using XPS shows that the ruthenium is present in a largely oxidic form.
DE 197 56 880 A1 describes the use of platinum/ruthenium alloy catalysts that have a reduced particle size, as compared with conventional catalysts, and thus provide increased carbon monoxide tolerance. However, an air-bleed of 3 vol.-% was also used there. Nothing is disclosed about the carbon monoxide tolerance in the case of reduced air-bleed or air-bleed-free operation. In addition, preparation using the colloids described is costly.
DE 44 43 701 C1 describes the use of monometallic or multimetallic supported catalysts. Here again, preparation requires the use of a prefabricated metal colloid. Nothing is disclosed about use as anode catalysts without an air-bleed.
EP 0 501 930 B1 describes quaternary alloys of platinum, nickel, cobalt and manganese as anode catalysts for phosphoric acid fuel cells (PAFC=phosphoric acid fuel cell), which have a high resistance to carbon monoxide at the high operating temperatures of a phosphoric acid fuel cell, 160xc2x0 C. to 200xc2x0 C. The alloy particles have a size of about 3 nm. At the high operating temperatures of the phosphoric acid fuel cell, however, the tendency of carbon monoxide to be adsorbed onto metal surfaces is lower in any case than at the low operating temperatures of a PEM fuel cell.
EP 0 549 543 B1 describes a process for preparing supported catalysts that contain highly disperse metal particles with average particle sizes of less than 2 nm. The process comprises reducing metal ions in a suspension of the support material, using a reducing agent in the presence of carbon monoxide, and simultaneous deposition on the support. The carbon monoxide present is adsorbed on the metal particles being deposited, which hinders further particle growth. After completion of the deposition process, the catalyst is washed and dried at a temperature of less than 100xc2x0 C. in a reducing atmosphere. The carbon monoxide is thus desorbed. In example 4, the preparation of a platinum/ruthenium catalyst on carbon with an average particle size for the platinum/ruthenium particles of 1.7 nm is described. However, this catalyst is not an alloy catalyst because adsorption of carbon monoxide on the metal particles during deposition prevents the formation of an alloy. Also, an alloy is not formed as a result of the subsequent thermal treatment at up to 100xc2x0 C. There is no data provided about the properties of this catalyst when used as an anode catalyst in a PEM fuel cell with a carbon monoxide-containing reformate gas.
Platinum/ruthenium catalysts on various support materials have been commercially available from the E-TEK, Division of DeNora N.A. Inc., 39 Veronica Avenue, Somerset N.J. 08873-6800 (USA) for some time. These are alloyed platinum/ruthenium catalysts with a noble metal loading between 5 and 60 wt.-% and a platinum/ruthenium atomic ratio of 1:1. Tests using a catalyst of this type (40 wt.-% PtRu on Vulcan XC 72) revealed an unsatisfactory tolerance to carbon monoxide, especially at concentrations of carbon monoxide of more than 100 ppm in the anode gas. As shown by surface analysis using XPS spectroscopy, the catalyst contains a considerable proportion of oxidised ruthenium at the surface.
M. Iwase and S. Kawatsu report, in an article, the development of carbon monoxide tolerant anode catalysts (M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, Volume 95-23, p. 12). In this article, the best results were produced with a platinum/ruthenium alloy catalyst in which the alloy was formed by special thermal treatment. Nevertheless, the voltage drop at a current density of 0.4 A/cm2 was still about 200 mV at a carbon monoxide concentration of 100 ppm. This is still far too high for practical operation.
Unsupported platinum/ruthenium catalysts as carbon monoxide tolerant anode catalysts for sulfuric acid fuel cells are described by L. W. Niedrach et al. (J. Electrochemical Techn. 5, 1967, p. 318). These materials consist of fine platinum/ruthenium alloy powders with high specific surface areas. They are prepared by the so-called ADAMS process by melting a mixture of hexachloroplatinic(IV) acid, ruthenium(III) chloride and sodium nitrate at 500xc2x0 C. However, this method of preparation raises problems with regard to protection of the environment and health (formation of large amounts of nitrous gases, working with liquid, corrosive melts).
Thus, the object of the present invention is to provide platinum-ruthenium catalysts that are characterised by improved tolerance to catalyst poisons, such as carbon monoxide in anode gas, in particular with small or zero air-bleeds. Furthermore, the PtRu catalyst should exhibit a good performance as anode catalysts in DMFCs. The method of preparation according to the invention should be far less damaging to the environment and to health than known processes.
The present invention provides a process for preparing an anode catalyst for fuel cells and the anode catalyst prepared therewith. In one embodiment the present invention provides a process for preparing a supported platinum-ruthenium catalyst on a powdered support material, said process comprising:
a. suspending the support material in water to form a suspension;
b. heating the suspension at most up to said solution""s boiling point;
c. after step b, while maintaining the same temperature and stirring, adding solutions comprised of hexachloroplatinic acid and ruthenium chloride to the suspension;
d. after step c, increasing the pH of the suspension to a value between 6.5 and 10 by adding an alkaline solution in order to precipitate hexachloroplatinic acid and ruthenium chloride in the form of sparingly soluble noble metal compounds;
e. after step d, adding one or more organic carboxylic acids and/or their salts to the suspension, and reducing the precipitated noble metal compounds by adding a reducing agent to form a catalyst; and
f. after step e, washing and drying the catalyst.